Not to sound rude, but that's where things start getting complicated. For one, the only people who know what Nintendo has planned is Nintendo themselves (4chan doesn't count). We've already seen proof that XCDE has some degree of 2 references, and there's even a rumor that Future Connected is actually meant to tie the two games even more (hence the name). Plus, the outcry of Rex's deconfirmation was a lot bigger than people give it credit for: even Japan was upset, and they're usually pretty chill with deconfirms. Combine that with the fact Sakurai himself admitted he would've added Rex in the base game had the circumstances been different, and Nintendo could pull a surprise on us and add Rex in after all.
Then again, this is Smash speculation, and nothing ever goes our way. But what I'm trying to say is, while you can keep petitioning for Elma (believe me, I won't stop you), you can't just assume Rex is dead in the water. Like I said earlier, there are too many unknowns.